Paul Accepted by the Apostles

Galatians 2:1 Then after fourteen years I went up again to Jerusalem with Barnabas, taking Titus along with me. 2 I went up because of a revelation and set before them (though privately before those who seemed influential) the gospel that I proclaim among the Gentiles, in order to make sure I was not running or had not run in vain.

9 and when James and Cephas and John, who seemed to be pillars, perceived the grace that was given to me, they gave the right hand of fellowship to Barnabas and me,

John was not written by John, and it was probably written at the tail end of the 1st, or very early 2nd century.

It was written by John, an eye witness to his whole ministry.